Exploration of Porphyrin-based Semiconductors for Negative Charge Transport Applications Using Synthetic, Spectroscopic, Potentiometric, Magnetic Resonance, and Computational Methods by Jeffrey Scott Rawson Department of Chemistry Duke University Date:_______________________ Approved: ___________________________ Michael J. Therien, Supervisor ___________________________ David N. Beratan ___________________________ Alvin L. Crumbliss ___________________________ Martin C. Fischer Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University 2014 i v ABSTRACT Exploration of Porphyrin-based Semiconductors for Negative Charge Transport Applications Using Synthetic, Spectroscopic, Potentiometric, Magnetic Resonance, and Computational Methods by Jeffrey Scott Rawson Department of Chemistry Duke University Date:_______________________ Approved: ___________________________ Michael J. Therien, Supervisor ___________________________ David N. Beratan ___________________________ Alvin L. Crumbliss ___________________________ Martin C. Fischer An abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University 2014 Abstract Organic π-conjugated materials are emerging as commercially relevant components in electronic applications that include transistors, light-emitting diodes, and solar cells. One requirement common to all of these functions is an aptitude for accepting and transmitting charges. It is generally agreed that the development of organic semiconductors that favor electrons as the majority carriers (n-type) lags behind the advances in hole transporting (p-type) materials. This shortcoming suggests that the design space for n-type materials is not yet well explored, presenting researchers with the opportunity to develop unconventional architectures. In this regard, it is worth noting that discrete molecular materials are demonstrating the potential to usurp the preeminent positions that π-conjugated polymers have held in these areas of organic electronics research. This dissertation describes how an extraordinary class of molecules, meso-to-meso ethyne-bridged porphyrin arrays, has been bent to these new uses. Chapter two describes vis-NIR spectroscopic and magnetic resonance measurements revealing that these porphyrin arrays possess a remarkable aptitude for the delocalization of negative charge. In fact, the miniscule electron-lattice interactions exhibited in these rigid molecules allow them to host the most vast electron-polarons ever observed in a π- conjugated material. Chapter three describes the development of an ethyne-bridged iv porphyrin-isoindigo hybrid chromophore that can take the place of fullerene derivatives in the conventional thin film solar cell architecture. Particularly noteworthy is the key role played by the 5,15-bis(heptafluoropropyl)porphyrin building block in the engineering of a chromophore that, gram for gram, is twice as absorptive as poly(3- hexyl)thiophene, exhibits a lower energy absorption onset than this polymer, and yet possesses a photoexcited singlet state sufficiently energetic to transfer a hole to this polymer. Chapter four describes synthetic efforts that expand the repertoire of readily available meso-heptafluoropropyl porphyrin building blocks. The findings suggest that the remaining challenges to the exploitation of these pigments will be overcome by a sufficiently firm grasp of their subtle electronic structures, and a willingness to eschew the customary strategies of chromophore assembly. v For Rachael, whose support proved to be the vital reagent that allowed me to reach this point. For Julius, who reminds me that there is more to life than one’s career. For Hilton, who was the first person to suggest that I could actually do this. vi Contents Abstract .......................................................................................................................................... iv List of Tables ................................................................................................................................. ix List of Figures ................................................................................................................................ x Acknowledgements ................................................................................................................... xvi 1. Introduction ............................................................................................................................... 1 1.1 Supermolecules ................................................................................................................. 1 1.2 Potentiometric scales and terminology ......................................................................... 2 1.3 n-Type materials ............................................................................................................... 4 1.4 Goals of the dissertation .................................................................................................. 5 2. Unsurpassedly vast electron-polarons in conjugated porphyrin arrays ........................... 8 2.1 Introduction ....................................................................................................................... 8 2.2 Results .............................................................................................................................. 10 2.3 Conclusion ....................................................................................................................... 18 2.4 Methods ............................................................................................................................ 19 2.4.1 Experimental methods .............................................................................................. 19 2.4.2 Computational methods ........................................................................................... 21 3. Tailoring Porphyrin-based Electron Accepting Materials for Organic Photovoltaics .. 26 3.1 Introduction ..................................................................................................................... 26 3.2 Results and Discussion .................................................................................................. 34 3.2.1 Supermolecular Chromophores Ar-Iso and Rf-Iso ............................................... 34 vii 3.2.2 Photovoltaic devices exhibiting acceptor-derived photocurrent ........................ 48 3.3 Conclusion ....................................................................................................................... 52 3.4 Methods ............................................................................................................................ 53 3.4.1 Experimental Methods .............................................................................................. 53 3.4.2. Theory and computational methods ...................................................................... 62 4. Synthesis of meso-(heptafluoropropyl)porphyrins ............................................................. 70 4.1 Introduction ..................................................................................................................... 70 4.2 Synthesis of 5-triisopropylsilylethynyl-10,15,20-tris(heptafluoropropyl)porphyrin MCQ ....................................................................................................................................... 72 4.3 Experimental study of porphyrin bromination regiochemistry guided by conceptual DFT calculations ............................................................................................... 80 4.3.1 Fukui functions for electron loss calculated for a series of (porphinato)zinc(II) complexes ............................................................................................................................ 80 4.3.2 Synthesis and bromination experiments for (5-triisopropylsilylethynyl-10,20- bis[heptafluoropropyl]porphinato)zinc(II) ..................................................................... 88 4.3.3 Conclusions from bromination experiments guided by conceptual DFT calculations .......................................................................................................................... 91 4.4 Progress toward ethyne-bridged oligomers of meso-perfluoroalkyl porpyrins ... 92 4.5 Methods ............................................................................................................................ 95 4.5.1 Experimental Methods .............................................................................................. 95 4.5.2 Computational Methods ......................................................................................... 106 4.5.3 NMR spectra ............................................................................................................. 108 References ................................................................................................................................... 117 Biography ................................................................................................................................... 140 viii List of Tables Table 1. Absorption Band Maxima, Energies, Extinction Coefficients, Full Widths at Half Maximum (FWHM), and Oscillator Strengths of PZn-Isoindigo